Tobacco addiction remains a leading preventable cause of premature deaths worldwide. In the United States, where approximately 47 million adults are current cigarette smokers, smoking-attributable health care expenditures and productivity losses exceed $190 billion annually. Abstinence rates of currently available smoking cessation treatments are modest. Understanding individual differences in vulnerability to tobacco dependence holds promise in identifying neurobiological substrates for new treatment targets and improving smoking cessation efficacy through tailored strategies. Recent seminal studies of nicotine self-administration in animals have indicated the critical role of the habenula a5 nicotinic receptor subunit in suppression of excessive nicotine intake. Ca. 60% of smokers carry the risk allele for non-synonymous coding SNP a5 subunit gene (rs16969968), which causes the substitution of an aspartic acid with asparagine and results in decreased function of the nicotinic receptor. This risk allele is associated with the number of cigarettes smoked per day, nicotine dependence and lung cancer. Our central hypothesis is that smokers with the a5 risk allele have blunted smoking behavior compensation in response to manipulation of cigarette nicotine delivery or supplemental delivery of nicotine via the skin patch. To address this hypothesis we propose three aims: Aim 1:To demonstrate that a5 risk allele carriers, relative to controls, will have greater increases in nicotine intake from cigarettes after switching to higher nicotine yield cigarettes; Aim 2: To prove that the reduction of nicotine intake from cigarettes while on the nicotine patch is less in smokers with the a5 risk allele than in smokers without these alleles; Aim 3: To show that a5 risk allele carriers demonstrate greater smoking compensation after switching to low nicotine yield cigarettes. To our knowledge, this project would be the first experimental study with human subjects to address the role of key a5 gene variants in regulating nicotine intake in response to nicotine manipulations. The project is also technologically innovative in the utilization of several novel approaches, including: assessment of nicotine dependence-relevant phenotypes by challenge; assessment of smoking behavior and nicotine intake in naturalistic environments; and use of cigarettes with controlled tar deliveries with varying nicotine yield. Successful completion of this study may provide critical data that inform scientific understanding of the role of the a5 subunit in nicotine dependence and how individual differences in sensitivity to nicotine may lead to strategies for individualizin smoking cessation treatments as well as for effective tobacco regulation policies, e.g., mandated reductions in tobacco nicotine content, currently being considered by FDA. The results can also provide justification for developing new treatments targeting nicotinic receptors containing the a5 subunit.